MWI is presented as stuttered dial tone and/or message waiting lamp on a compatible telephone.
Since GV don't really have an API to support MWI, the current method is only through
periodic polling. For this we use a simple backoff strategy, try to balance between
bandwidth consumption and responsiveness of the MWI. I would just say it's not as
robust as a qualified SIP-based ITSP in this aspect.
Based on your feedback we will try to improve the algorithm over time.
Ideally, eventually, GV would have a more real-time VM notification mechanism to support
devices like ours. For now this is all we can offer.